"We were terrified when we started hearing gunshots echoing ... but everywhere became calm about an hour later. The soldiers informed us that it was the Boko Haram terrorists that came back", said Aji Kaumi.

He and Mohammed Umara said 11 civilians were killed on the outskirts of Gamboru.

They and other fearful residents have crossed back to the relative safety of the Cameroonian town of Fotokol, from which they spoke to The Associated Press by telephone Thursday night.

It was the first reported attack on a town previously occupied by Boko Haram since a regional offensive was launched by troops from Chad seven weeks ago. Nigerian officials this week said they have taken back 38 northeastern towns and forced Boko Haram out of two of three states and freed most of Borno state.

The extremists had seized Gamboru in November and were dislodged Feb. 4.

The offensive comes as Nigerians prepare for a closely contested March 28 presidential election.

Nigeria says the multinational push aims to eradicate Boko Haram. But a French diplomat says a more realistic goal is to reduce Boko Haram to what it used to be — an extremist group holding no territory. The high-ranking diplomatic briefed journalists in Paris on condition of anonymity earlier this month.

France brought about the regional cooperation with a summit in Paris last year that eased strained relations between Nigeria and its smaller, francophone neighbors amid growing concern as Boko Haram began launching attacks across Nigeria's borders.

France has sent military advisers to Niger and its Chadian-based aircraft are on scouting missions to help the effort.

The death toll from the nearly six-year Islamic uprising mounted exponentially last year with some 10,000 people killed and more than 1.5 million driven from their homes, according to the Washington-based Council on Foreign Relations.